1. Field of the Invention
The present invention relates to quantum key distribution for encrypted communications.
2. Description of the Related Art
The following disclosures are incorporated by reference in their entirety: Experimental Single Qubit Quantum Secret Sharing, C. Schmid, P. Torjek, M. Bourennane, C. Kurtsiefer, M. Zukowski, and W. Weinfurter, Phsy. Ref. Lett. 95, 230505 (2005); and Quantum Cryptography, N. Gisin, G. Ribordy, W. Tittel, and H. Zbinden, Rev. Mod. Phys. 74, 145 (2002).
Quantum key distribution (QKD) involves the application of quantum physics to generate and distribute a shared key for encrypted communications. The security of the key is based on features of quantum physics, rather than assumptions regarding computationally difficult problems. In particular, QKD exploits quantum phenomena to enable communications that can only be intercepted by violating known laws of physics. In recent years QKD systems have been physically demonstrated to be invulnerable to eavesdropping attacks, and commercial QKD systems are presently available for point-to-point secure communication over fiber optic cable.
A classical QKD system is generally configured for two clients, a sender and a receiver. The sender and the receiver share a random series of bits known only to them, which are then used as a secret key for the encryption and decryption of plaintext. For example, the sender sets the quantum state (e.g., polarization state) of binary information, makes a record of how it set the quantum state (e.g., rectilinear basis or diagonal basis), and transmits the information. The receiver measures the quantum state of the binary information and records how it measured the quantum state. The measured quantum state (e.g., 0°, 45°, 90°, 135°) depends on how the receiver measured the binary information. The sender and the receiver share how the binary information was sent and measured across a public channel, and discard the bits that were not sent and measured in the same basis, leaving roughly half of the measured bits as the secret key.
Extending the classical QKD system to more than two clients normally requires a separate QKD link for each pair of nodes. For example, FIG. 1 illustrates an extension of a classical QKD system above for five additional clients. An extension of the classical QKD system of this nature would require significant QKD resources, however, rendering it too expensive for multi-client communications. The disclosure of Schmid et al includes a modified multi-party QKD system in which all but one of the QKD clients confer to privately share their actions to reconstruct a secret. However, this system requires a large secret meeting in order to deduce the actions of the additional client, thereby contributing cost and complexity to the multi-party QKD system.